use strict;
use warnings;
 
my $magic = 600851475143;
 
sub largestprimef($);
sub max($$);
 
print largestprimef($magic);
sub largestprimef($)
{
my $n = shift;
 
my $i;
return largestprimef(max(2, $n/2) if($n % 2 == 0)); #UPDATE: There was a bug here where I didn't missed the recursive call. Corrected 20th April 2009.
my $sn = int(sqrt($n));
 
for ($i = 3; $i <= $sn; $i += 2)  {    if($n % $i == 0)    {     last;    }   }   if($i > $sn) #loop ran over, means the number is prime
{
return $n;
}
else
{
return max($i, largestprimef($n/$i));
}
}
 
sub max($$)
{
return (sort { $a <=> $b }(@_))[1];
}

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